(1) Field of the Invention
The present invention relates to the field of earpieces (such as earplugs, ear phones, and earbuds) and more particularly to earpieces made of polyurethane foam.
(2) Description of the Related Art
Earpieces are earplugs that fit into the ear canal, with the function of protecting the user from unwanted external noise. Earpieces may also be earplugs with a sound passageway through the acoustic seal that fit into the user's ear canal. Earpieces are part of a larger group of headpieces that fit in, on, or around the ear. Headpieces (including earpieces, earbuds and stereo pieces) are pairs of small speakers, or less commonly single speakers, with ways of holding them close to or in a user's ears, and with a means of connecting them to a sound source such as an audio amplifier, radio, CD player, MP3 player, or any other communications or sound-modifying device. They can consist of any audio or sound device or component that attenuates, transmits or modifies sound and that is designed to be used in, on, or around the ear. Although earpieces can be made from a variety of materials, the vast majority of earplugs and a growing number of other earpieces are made of polyurethane foam. Such foam is comfortable, non-toxic, manufacturing-friendly and highly effective as a sound-attenuation material, providing the user with a good fit and acoustic seal, as required.
Current art for the production of molded flexible polyurethane foams consist of mixing isocyanates (A-side) with isocyanate-reactive hydrogen-containing compounds (B-side) in the presence of a foaming agent such as water or carbon dioxide. The resulting exothermic reaction produces flexible polyurethane foam.
Prior to reaction, the A-side (isocyanates) and the B-side (hydrogen-containing compounds) are mixed and the resulting blend is able to flow. At this stage the mixture is usually dispensed into the cavity of a mold. The chemicals react and expand into foam and fill the cavity of the mold. The mold can be an open mold, a closed mold, or a reaction-injection mold. For maximum process stability the exothermic reaction best proceeds under controlled temperature conditions. Chemical reaction speeds and foam physical properties can also be altered through the use of a wide variety of catalysts and additives. Such additives may include: surfactants, catalysts, chain-extenders, cross-linkers, pH modifiers, blowing agents, inorganic and organic fillers, coefficient-of-friction modifiers, anti-microbial agents, mineral oil, vegetable oil, and others.
A special polyurethane foam for making earpieces has been developed by Sperian Hearing Protection, LLC. The A-side component of the formulation consists of a prepolymer. The B-side is a hydrogen-providing compound, along with other property-modifying agents that when reacted together produce a slow-recovery polyurethane foam at low cost. This special formulation is used to make earpieces consistent with those described in the previous U.S. Pat. No. 4,774,938.
Foam formulators are always searching for ways to manufacture foams at low cost. While the use of recycled material is common in other plastics, especially thermoplastics, the use of recycled pulverized polyurethane foam (RPPF) has not, to date, been successful in the manufacture of earpieces. The reasons for this include:
difficulty in keeping the pulverized foam in suspension;
difficulties with pumping, mixing and metering a blend containing particles;
incompatibility of the recycled foam with the polyurethane foam made;
difficulty in integrating the RPPF into the polymer matrix;
uncleanliness of the recycled foam; and,
inability to maintain critical properties such as: attenuation, comfort, strength, recovery time, density, particle dispersion, impression force depression, reaction time, skin formation, rise time, cream time, coefficient of friction, cell size, strut and window configuration in cell walls, open and closed cell distribution, colorant absorption, color fastness, flame resistance, u.v. inhibition, and antioxidant properties.
Development of a method for making polyurethane foam earpieces more inexpensively by incorporating recycled foam into the formulation represents a great improvement in the field of earpieces manufacture and satisfies a long-felt need of the earpiece manufacturer.